I run grunge guard on the fat bike, especially during our pacific northwest wet season. With the current front guard, anything on my front rack still gets a pretty good grunging. One option I have used is to carry a dry bag to put things in and then place on my rack. While not a big inconvenience, I wanted to try to add a grunge guard so I could just place whatever and not worry about it getting mucky.
Having extra, rear, seat-post mounted fenders around, I decided to sacrifice one for this experiment. I trimmed up the rear edge to meet up with my existing guard. then measured the forward section to end at the front edge of my rack. Next I marked the cross rail points and drilled holes for my zip-tie attachment points. Some minor edge clean up and we are ready for some testing.